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(subclass Pulmonata), any of various land and freshwater (a few marine) snails, class Gastropoda, that breathe by means of a "lung"-a saclike modification of the mantle cavity. The slugs lack an external shell, but most pulmonates have a spiral shell that may be attenuated or flattened. They are anatomically more advanced than other snails: all are detorted (bilaterally symmetrical, with an unlooped nerve cord) as adults, and the auricle of the heart is anterior to the ventricle. All are hermaphroditic-i.e., with both male and female reproductive organs in one animal.